The present invention relates to a process for the production of string-like porous materials, and more particularly, to a process for producing string-like porous materials of polytetrafluoroethylene, having fine pore diameters and high porosities.
The term "string-like material" is used herein to refer to a solid or hollow cylindrical material, e.g., a tube and a rod.
Japanese Patent Publication No. 13560/67 and U.S. Pat. No. 3,953,566 disclose methods of producing string-like porous materials of polytetrafluoroethylene. It has been found, however, that these methods suffer from disadvantages in that they can provide only string-like materials having large pore diameters. In accordance with these processes if the pore diameter is decreased to 0.5 micron or less, the porosity of the string-like material will be seriously reduced.
A method of stretching polytetrafluoroethylene string-like materials is described in U.S. Pat. No. 3,664,915. In accordance with this method, a heating furnace is placed intermediate between two pairs of pinch rolls which are not heated. By running the two pairs of pinch rolls at different rates of rotation, the string-like material is stretched while it is passed through the heating furnace. It has been found, however, that when the method is used to stretch string-like materials, a considerably large temperature difference will occur between the surface of the string-like material and the inside, making it difficult to achieve uniform stretching.
It is believed that in order to achieve uniform stretching, it is desirable to heat the string-like material in such a manner that the temperature of the inside thereof is equal to that of the surface before the stretching force is exerted on the string-like material, and to apply a given compression force all around the string-like material after the stretching force is exerted.
The method disclosed in U.S. Pat. No. 3,664,915, however, does not meet either of the above two requirements.